1. Field of the Invention
The present invention relates to a motor used for, for example, in-car electrical equipment and game consoles and, in particular, to a motor incorporating a positive temperature coefficient thermistor to control an overcurrent by sensing the temperature of the motor, to a motor with encoder using such a motor, and to a multi-direction input device using such a motor.
2. Description of the Related Art
A motor is widely used in various fields, such as in-car electrical equipment and game consoles. For example, a motor can be used for a motor with encoder in which the rotation of a code plate coupled to a rotation shaft of the motor is sensed by a photo interrupter (refer to, for example, Japanese Examined Utility Model Registration Application Publication No. 6-44295, in particular, pages 1 to 2 and FIG. 2). Also, a motor can be used for a multi-direction input device in which two orthogonally disposed motors function as driving sources when pivotally operating an operation lever (refer to, for example, Japanese Unexamined Patent Application Publication No. 2002-108471, in particular, pages 5 to 7 and FIG. 4).
Among such motors used in various fields, a compact motor is known in which an opening of a cylindrical casing closed at one end is covered by a holder having the same shape and the holder holds a positive temperature coefficient thermistor (refer to, for example, Japanese Unexamined Utility Model Registration Application Publication No. 7-44602, in particular, pages 4 to 5 and FIG. 1). The positive temperature coefficient thermistor (hereinafter simply referred to as a “PTC”) is a plate-shaped device composed of two laminated electrodes with a resistance material therebetween. The internal electrical resistance of a PTC rapidly increases when a certain temperature is exceeded. Accordingly, in a motor incorporating a PTC, when the motor is continuously overloaded or the rotation is forced to stop, an overcurrent flows in the motor, and therefore, the temperature inside the motor rises and the temperature of the PTC also rises. This rapidly increases the internal resistance value. Thus, an electrical current supplied to the motor rapidly decreases. As a result, the motor can be protected against overheating.
On the other hand, in the above-described known motors, a cylindrical casing closed at one end surrounds a commutator attached to a rotation shaft. The opening of the casing is covered by a holder having the same shape and the holder holds a PTC. Accordingly, a shell of the motor is composed of a cylinder in which the casing and the holder are integrated. This structure prevents the size of a product incorporating the motor from being reduced. For example, when this motor is applied to a motor with encoder and a code plate coupled to the rotation shaft is not sufficiently distant from an end of the holder, a photo interrupter hits against the holder. Accordingly, the total length of the motor with encoder inevitably increases in its axis direction. Also, when this motor is applied to the above-described multi-direction input device and two orthogonally arranged motors are not sufficiently distant from each other at the intersection thereof, holders integrated with the bottom ends of the motors hit against each other. This also increases the size of the multi-direction input device.